Water-dilutable binders, containing latent primary amino groups, for cationic electrocoating finishes, and a process for their preparation

ABSTRACT

The invention relates to water-dilutable binders for cationic electrocoating finishes, the said binders containing latent primary amino groups. In order to prepare the binders, an epoxy resin is reacted with a polyamine, which in addition to a sterically hindered primary amino group also contains at least one primary amino group blocked by reaction with a ketone or aldehyde, and, if appropriate, further primary or secondary amines.

This application is a division of application Ser. No. 474,259, filedFeb. 5, 1990 now U.S. Pat. No. 4,980,399 which is a continuation of07/185,147 filed Apr. 22, 1988 now abandoned which is a divisional of07/010,196 filed Jan. 23, 1987 now U.S. Pat. No. 4,762,903.

The present invention relates to water-dilutable binders for cationicelectrocoating finishes, which binders contain latent primary aminogroups and are based on reaction products of epoxy resins andpolyamines, which contain at least one blocked primary amino group, and,if required, further primary and/or secondary amines.

Cationic electrocoating is a coating process frequently used, inparticular, for priming, and in which water-dilutable synthetic resinscarrying cationic groups are applied onto electrically conductivearticles with the aid of direct current.

The use of modified epoxy resins as binders for cationic electrocoatingfinishes is known. Water-dilutable modified epoxy resins can beobtained, for example, by reacting polyepoxide compounds with aminesand, if appropriate, further modifiers (U.S. Pat. No. 4,104,147).

Modified epoxy resins prepared in this manner contain secondary andtertiary amino groups. It is assumed that the low basicity of thetertiary amino groups, in particular, is responsible for the fact thatthe binders obtained from reaction products of polyepoxide compoundswith amines are only poorly dispersible in water and give aqueoussystems having a low pH.

In order to overcome these difficulties, attempts were made to introduceprimary amino groups into the epoxide binders. This objective wasachieved by reacting polyepoxide compounds with a (poly)ketiminecontaining secondary amino groups or containing hydroxyl groups, andthen liberating the protected primary amino group or groups byhydrolysis. (U.S. Pat. No. 3,947,339, U.S. Pat. No. 4,017,438, U.S. Pat.No. 4,104,147 and U.S. Pat. No. 4,148,772).

It has been found that the water dispersibility and the electricaldeposition properties of the modified epoxy resins are substantiallymore advantageously affected by increasing the content of primary aminogroups than by increasing the content of tertiary amino groups.

Since low molecular weight amines have an adverse effect on the surfaceproperties of the electrically deposited layer, care should be taken toensure that the binders are free of unconverted amine derivatives.

When primary amino groups are introduced via ketimines containingsecondary amino groups, tertiary amino groups, which are not veryadvantageous, are formed, whereas when ketimines containing hydroxylgroups are used, lower reactivity and selectivity have to be accepted asdisadvantages.

It is an object of the present invention to provide water-dilutablemodified epoxy resins which have a very high content of primary aminogroups and are free of low molecular weight amines.

This object is achieved, according to the invention, if the binders areprepared from

A) an epoxy resin and

B) a reaction product obtained by reacting a ketone or aldehyde with apolyamine which contains not only a sterically hindered primary aminogroup but also at least one further primary amino group, and, ifrequired, further

C) primary or secondary amines, and the primary amino groups are thenliberated by hydrolysis.

The particular advantages achieved with the invention are that, inaddition to the raw material base being extended, the componentcontaining the latent primary amino groups is linked to the epoxy resinvia a sterically hindered primary amino group, with the result that

instead of the tertiary amino groups, which are not very advantageous,more strongly basic secondary amino groups containing an active H atomare formed, and

because of the high reactivity and selectivity of the primary aminogroup, rapid and quantitative conversions are achieved.

All compounds which contain at least one reactive epoxide group,preferably two reactive epoxide groups, can be employed as component A.

The molecular weight of component A is preferably in the range from 350to 2000.

Preferred epoxy resins are polyglycidyl ethers of polyphenols, theseethers being prepared from polyphenols and epihalohydrins. For example,the following compounds can be used as polyphenols:2,2-bis-(4-hydroxyphenyl)-propane, 4,4'-dihydroxybenzophenone,1,1-bis-(4-hydroxyphenyl)-ethane, 1,1-bis-(4-hydroxyphenyl) -isobutane,2,2-bis-(4-hydroxy-tert.-butyl-phenyl)-propane,bis-(2-hydroxynaphthyl)-methane, 1,5-dihydroxynaphthalene and phenolicnovolak resins.

Polyglycidyl ethers of polyhydric alcohols such as ethylene glycol,diethylene glycol, triethylene glycol, 1,2-propylene glycol,1,3-propylene glycol, pentane-1,5-diol, hexane-1,2,6-triol, glycerol and2,2-bis-(4-hydroxycyclohexyl)-propane, are also suitable.

Polyglycidyl esters of polycarboxylic acids, such as, for example,oxalic acid, succinic acid, glutaric acid, terephthalic acid,napthalene-2,6-dicarboxylic acid, dimerized linoleic acid, etc., canalso be used. Typical examples are glycidyl adipate and glycidylphthalate.

Hydantoin epoxides and polyepoxide compounds which are obtained byepoxidation of an olefinically unsaturated alicyclic compound are alsosuitable.

It is also possible to use epoxy resins which have been modified byreaction with

A) Low molecular weight and/or high molecular weight compoundscontaining carboxyl groups, such as, for example, saturated orunsaturated monocarboxylic acids for example benzoic acid, linseed oilfatty acid, 2-ethylhexanoic acid or Versatic acid), aliphatic,cycloaliphatic and/or aromatic dicarboxylic acids of various chainlengths (for example adipic acid, sebacic acid, isophthalic acid, ordimeric fatty acids), hydroxyalkylcarboxylic acids (for example lacticacid or dimethylolpropionic acid) and polyesters containing carboxylgroups, and/or

B) Low molecular and/or high molecular weight compounds containing aminogroups, such as diethylamine or ethylhexylamine, or diamines possessingsecondary amino groups, for example N,N'-dialkylalkylenediamines, suchas dimethylethylenediamine, N,N'-dialkyl polyoxyalkyleneamines, such asN,N'-dimethylpolyoxypropylenediamine, cyanoalkylated alkylenediamines,such as bis-N,N'-cyanoethylethylenediamine, cyanoalkylatedpolyoxyalkyleneamines, such asbis-N,N'-cyanoethylpolyoxypropylenediamines, polyaminoamides, such asversamides, or the reaction product of one mole of diaminohexane withtwo moles of monoglycidyl ethers or monoglycidyl esters, especiallyglycidyl esters of α-branched fatty acids, such as Versatic acid, and/or

C) Low molecular weight and/or high molecular weight compoundscontaining hydroxyl groups, such as neopentyl glycol, bis-oxyethylatedneopentyl glycol, neopentyl glycol hydroxypivalate,dimethylhydantoin-N,N'-diethanol, hexane-1,6-diol, hexane-2,5-diol,1,4-bis-(hydroxymethyl)-cyclohexane,1,1-isopropylidene-bis-(p-phenoxy)-2-propanol, trimethylolpropane,pentaerythritol or amino alcohols, such as triethanolamine,methyldiethanolamine or alkylketimines containing hydroxyl groups, suchas aminomethylpropane-1,3-diol-methyl-isobutyl-ketimine ortris-(hydroxymethyl)-aminomethane-cyclohexanone ketimine, and polyglycolethers, polyester polyols, polyether polyols, polycaprolactone polyolsand polycaprolactam polyols of various functionalities and molecularweights and/or

D) methyl esters of saturated or unsaturated fatty acids, which aretransesterified with hydroxyl groups of the epoxy resins in the presenceof sodium methylate and/or

E) compounds containing thiol groups and/or

F) sulfide/acid mixtures and/or

G) phosphine/acid mixtures

and still possess free epoxide groups.

The modified epoxy resins disclosed in CA 1179443 are particularlypreferably used.

In preparing component B, the sterically unhindered primary amino groupsof the polyamine are blocked by reaction with a ketone or an alkdhyde.The use of the ketones, which are less reactive toward the aldehydes,have the advantage that the ketones eliminated after the hydrolyticliberation of the primary amino groups do not give rise to any technicalproblems when present in the electrocoating bath. Apart from theadvantage of higher reactivity, use of aldehydes has the disadvantagethat the eliminated aldehydes are expected to be oxidized in the coatingbath to the corresponding carboxylic acids when the residence time islong, and consequently technical difficulties may occur.

Ketones of the general structural formula I are suitable for thepreparation of component B. ##STR1## R¹ and R² are organic groups whichare chemically inert during ketimine formation. R¹ and R² are preferablyalkyl radicals of 1 to 4 carbon atoms. It is frequently advantageous touse a ketone which readily distills over with water.

Preferred examples of ketones include acetone, methyl ethyl ketone,diethyl ketone, methyl propyl ketone, methyl isopropyl ketone, methyln-butyl ketone, methyl isobutyl ketone, ethyl isopropyl ketone,cyclohexanone, cyclopentanone and acetophenone. Particularly preferredketones are acetone, methyl ethyl ketone and methyl isobutyl ketone.

Compounds of the general structural formula II are suitable aldehydes.##STR2## R³ is an organic group, preferably an alkyl or aryl group. Itis advantageous to use an aldehyde which readily distills over withwater. Typical examples of aldehydes which may be used are acetaldehyde,propionaldehyde, butyraldehyde, isobutyraldehyde and benzaldehyde.Acetaldehyde is particularly preferred.

In addition to containing a sterically hindered primary amino group, thepolyamines which may be used for the preparation of component B mustalso contain at least one further primary amino group. Stericallyhindered primary amino groups are understood as meaning primary aminogroups which, because of steric effects, are shielded from attack bybulky reactants.

Preferred polyamines are unsubstituted or substituted alicyclic oraliphatic polyamines which, in addition to a sterically hindered primaryamino group, also contain at least one further primary amino group.Unsubstituted or substituted alicyclic or aliphatic polyamines which, inaddition to a primary amino group sterically hindered by one α-alkylgroup or by two α-alkyl groups, also contain at least one furtherprimary amino group are particularly preferred.

Polyamines, which are based on formula II below ##STR3## wherein R⁴ isalkyl of 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, R⁵ isH or R⁴, R⁶ is unsubstituted or substituted alkylene of 1 to 20 carbonatoms, preferably --(CH₂)_(m) -- where m is 1-10, and n is 1, are veryparticularly preferred.

Typical examples of polyamines which are suitable for the preparation ofcomponent B are trimethylhexamethylene-1,6-diamine, isophoronediamine,propane-1,2-diamine and 1,2-diamono-2-methylpropane.

In a preferred process for the preparation of component B, the carbonylcomponent and the polyamine in a suitable solvent (for example xylene,toluene or n-hexane) are heated, and the water of reaction is removed byazeotropic distillation. If about 1 mole of ketone or aldehyde isemployed per mole of primary amino groups to be blocked, as a rule allsterically unhindered primary amino groups are blocked.

The primary or secondary amines used as component C should as far aspossible be water-soluble compounds. For example, mono- anddialkylamines, such as methylamine, ethylamine, propylamine, butylmine,dimethylamine, diethylamine, dipropylamine, methylbutylamine, etc., canbe used. Alkanolamines, such as, for example, methylethanolamine anddiethanolamine, are also suitable. Dialkylaminoalkhylamines, such as,for example, dimethylaminoethylamine and diethylaminopropylamine, canalso be employed.

Low molecular weight amines are used in most cases, but it is alsopossible to employ relatively high molecular weight monoamines,particularly when it is intended to increase the flexibility of theresin by incorporating such amines. In a similar manner, it is alsopossible to use mixtures of low molecular weight amines and highermolecular weight amines for modifying the resin properties.

The invention also embraces a process for the preparation of thebinders, wherein

A) an epoxy resin is reacted with

B) a reaction product obtained by reacting a ketone or aldehyde with apolyamine, which in addition to a sterically hindered primary aminogroup also contains at least one further primary amino group, and, ifrequired, also with a further

C) primary or secondary amine,

and the primary amino groups are liberated by hydrolysis.

The reaction of the epoxy resin with component B is frequentlyexothermic. In order to complete the reaction, it may be useful in somecases to heat the reaction mixture to 40°-130° C. The reactionconditions should be chosen so that the blocked primary amino groups areliberated only when there is no longer any possibility of the formationof higher molecular weight products. The primary amino group isliberated hydrolytically on dilution with water.

The reaction of the epoxy resin with component C is often exothermic andmay be carried out either before or after the reaction with component B.Depending on the desired course of the reaction, it is advisable toincrease the reaction temperature to 130° C.

By determining the epoxide equivalent weight in each case, the course ofthe reactions of the epoxy resin with components B and C can bemonitored.

Water-dilutability of the binders is achieved by neutralization of theamino groups with water-soluble acids (for example formic acid, aceticacid or phosphoric acid).

The binders prepared according to the invention can be crosslinked bythe addition of crosslinking agents by conventional methods, or can beconverted to self-crosslinking systems by chemical modification. Theself-crosslinking system can be obtained, for example, by reacting thebinder with a partially blocked polyisocyanate which possesses onaverage one free isocyanate group per molecule and whose blockedisocyanate groups are unblocked only at elevated temperatures.

Frequently used methods for the crosslinking of binders have beenpublished in, for example, the following documents: GB-1 303 480-A, EP12 463 and U.S. Pat. No. 4,252,703, U.S. Pat. No. 4,364,860 and GB 1 557516-A.

Examples of suitable aminoplast crosslinking agents are the hexamethylether of hexamethylolmelamine, the triethyltrimethyl ether ofhexamethylolmelamine, the hexabutyl ether of hexamethylolmelamine andthe hexamethyl ether of hexamethylolmelamine, and polymeric butylatedmelamine/formaldehyde resins. Alkylated urea/formaldehyde resins mayalso be used. Blocked polyisocyanates, too, may be employed ascrosslinking agents. In the invention, it is possible to use anypolyisocyanates in which the isocyanate groups have been reacted with acompound so that the blocked polyisocyanate formed is stable towardhydroxyl groups at room temperature but reacts at elevated temperatures,as a rule in the range from about 90° to about 300° C. In thepreparation of the blocked polyisocyanates, any organic polyisocyanatessuitable for the crosslinking reaction can be used. The isocyanateswhich contain about 3 to about 36, in particular about 8 to about 15,carbon atoms, are preferred. Examples of suitable diisocyanates aretrimethylene diisocyanate, tetramethylene diisocyanate, pentamethylenediisocyanate, hexamethylene diisocyanate, propylene diisocyanate,ethylethylene diisocyanate, 2,3-dimethylethylene diisocyanate,1-methyltrimethylene diisocyanate, 1,3-cyclopentylene diisocyanate,1,4-cyclohexylene diisocyanate, 1,2-cyclohexylene diisocyanate,1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-toluylenediisocyanate, 2,6-toluylene diisocyanate, 4,4'-diphenylene diisocyanate,1,5-naogthylene diisocyanate, 1,4-naphthylene diisocyanate,1-isocyanatomethyl-5-isocyanato-1,3,3-trimethylcyclohexane,bis-(4-isocyanatocyclohexyl)-methane, bis-(4-isocyantophenyl)-methyl4,4'-diisocyanatodiphenyl ether and2,3-bis-(8-isocyanatooctyl)-4-octyl-5-hexylcyclohexene.

It is also possible to use polyisocyanates of higher isocyanatefunctionality. Examples of these are tris-(4-isocyanatophenyl)-methane,1,3,5-triisocyanatobenzene, 2,4,6-triisocyanatotoluene,1,3,5-tris-(6-isocyanatohexyl)-biuret,bis-(2,5-diisocyanato-4-methylphenyl)-methane and polymericpolyisocyanates, such as dimers and trimers of diisocyanatotoluene. Itis also possible to use mixtures of polyisocyanates. Other organicpolyisocyanates which are suitable as crosslinking agents for theinvention may be prepolymers which are derived from, for example,polyol, including a polyether polyol or a polyester polyol.

Any suitable aliphatic, cycloaliphatic or aromatic alkyl monoalcoholscan be used for blocking the polyisocyanates. Examples of these arealiphatic alcohols, such as methyl, ethyl, chloroethyl, propyl, butyl,amyl, hexyl, heptyl, octyl, nonyl, 3,3,5-trimethylhexyl, decyl andlauryl alcohol; cycloaliphatic alcohols, such as cyclopentanol andcyclohexanol; and aromatic alkyl alcohols, such as phenylcarbinol andmethylphenylcarbinol.

Other suitable blocking agents are hydroxylamines, such as ethanolamine,and oximes, such as methyl ethyl ketoxime, acetone oxime andcyclohexanone oxime. In suitable proportions, the stated polyisocyanatesand blocking agents can also be used for the preparation of thepartially blocked polyisocyanates described above.

Generally used additives, such as, for example, coalescing solvents,pigments, surfactants, crosslinking catalysts, antioxidants, fillers andantifoams, can be added to the aqueous coating compositions preparedusing the binders according to the invention.

Aqueous systems prepared using the binders according to the inventionare particularly suitable for the cationic electrocoating process;however, they may also be employed in conventional coating processes.For example, metals which may or may not have been pretreated, such asiron, steel, copper, zinc, brass, magnesium, tin, nickle, chromium andaluminum, as well as impregnated paper and other electrically conductivesubstrates may be used as substrates for coating.

The examples which follow illustrate the invention in more detail. Partsand percentages are by weight, unless expressly stated otherwise.

I. PREPARATION OF A DIAMINO-MONOKETIMINE

853.3 g of 1,2-propylenediamine (11.5 moles), 1153.9 g of methylisobutyl ketone (11.5 moles) and 200 g of n-hexane are weighed into areaction vessel equipped with a stirrer, an internal thermometer, aninert gas feedline and a water separator and connected to an upstreamRaschig column, and the mixture is heated to 90° C., at which point thefirst water-containing distillate is obtained. As the reactionprogresses, the temperature is increased stepwise to 105° C. and kept atthis value until no further water of reaction separates off and thetheoretical amount of 200 g of water is approximately reached. Theproduct is then stored under nitrogen. The solution has an amineequivalent weight of 87.5 and a theoretical solids content of 89.3%.

Analysis of the distillate gives a 1,2-propylenediamine loss of 2.8%(determined titrimetrically) and a methyl isobutyl ketone loss of 0.05%(determined by gas chromatography).

II. PREPARATION OF THE BINDER I

2004 parts of an epoxy resin based on bisphenol A and having an epoxideequivalent weight of 501 and 312 parts of xylene are initially taken ina suitable reactor and heated to 115° C. The resulting traces of waterare removed by separating them off under slightly reduced pressure. 105parts of hexylglycol are then added, and the mixture is cooled to 70° C.157 parts of diethanolamine are then added dropwise in the course of 30minutes, and the mixture is kept at this temperature for 2 hours.Thereafter, 630 parts of a reaction product of 1 mole ofhexamethylenediamine and 2 moles of Cardura E 10 (commercial productfrom shell) are added, and the temperature is increased to 120° C. Afterone hour, the mixture is cooled and, during this procedure, 195 parts ofsecondary butanol are added. When the temperature has reached 70° C., 84parts of the monoketimine (I) described above are added, and thetemperature is increased to 80° C. in the course of 1 hour. After afurther 2 hours, the mixture is cooled and the product is discharged. Aclear resin solution having the following characteristics is obtained:

Solids content (1 hour at 130° C.): 82.5%

MEQ base: 1.5 meq/g

Viscosity (40% strength in propyleneglycol monomethyl ether): 3.0 dPas

III. PREPARATION OF A BINDER II

1827 parts of a commercial bisphenol A epoxy resin (epoxide equivalentweight 188), 212 parts of dodecylphenol and 63 parts of xylene areinitially taken in a suitable reactor and heated to 130° C. 12 parts ofdimethylbenzylamine are added and, after a brief exothermic reaction,the temperature is kept at 132° C. When an epoxide equivalent weight of400 has been reached, a further 298 parts of dodecylphenol are added,and the reaction is continued until an epoxide equivalent weight of 720is reached. Thereafter, the mixture is diluted with 229 parts of xyleneand 162 parts of hexylglycol and cooled at the same time. At 85° C., 255parts of diethanolamine are added and the temperature is kept at thislevel until the total content of epoxide and amine has reached 1.28meq/g. 79 parts of the monoketimine from Example I are then added. Thetemperature is increased to 115° C. in the course of 2 hours and kept atthis level for a further hour, after which the mixture is cooled for ashort time and the product then discharged. The product has thefollowing characteristics:

Solids content (1 hour at 130° C.): 85.5%

MEW base: 1.45 meq/g

Viscosity (40% strength in propyleneglycol monomethyl ether): 1.0 dPas

PREPARATION OF CROSSLINKING AGENTS

To obtain highly resistant coatings, it is advantageous to cure thebinders by means of a chemical reaction by admixing a crosslinkingagent. Examples of crosslinking agents with which the binders accordingto the invention form stable mixtures at room temperature and undergocrosslinking on heating are described below.

CROSSLINKING AGENT I

A blocked isocyanate crosslinking agent (polyurethane crosslinkingagent) is prepared as described in German Laid-Open Application DOS2,701,002, Example I, by adding 218 parts of 2-ethylhexanol slowly to291 parts of an 80:20 isomer mixture of 2,4-/2,6-toluylene diisocyanatewhile stirring and in a nitrogen atmosphere, the reaction temperaturebeing kept below 38° C. by external cooling. The mixture is kept at 38°C. for a further half an hour and then heated to 60° C., after which 75parts of trimethylolpropane and then, as a catalyst, 0.08 part ofdibutyltin dilaurate are added. After an initial exothermic reaction,the mixture is kept at 121° C. for 1.5 hours until all the isocyanategroups have essentially been consumed, which can be detected from theinfrared spectrum. The mixture is then diluted with 249 parts ofethylene glycol monoethyl ether.

CROSSLINKING AGENT II

A polyester crosslinking agent is prepared as described in EuropeanPatent 0 040 8687, Example 2d): 192 parts of trimellitic anhydride and500 parts of glycidyl Versatate, having an epoxide equivalent weight of250, are mixed, and the mixture is heated to 100° C., while stirring. Anexothermic reaction occurs and the temperature increases to 190° C.After the mixture has been cooled to 140° C., 2 parts ofbenzyldimethylamine are added. The mixture is kept at 140° C. for 3hours. A viscous, clear product is formed, and is additionally dilutedwith 295 parts of ethylene glycol monobutyl ether.

V. PREPARATION OF AQUEOUS DISPERSIONS I -III

To prepare the aqueous dispersions, the binder is initially takentogether with butylglycol, an antifoam, glacial acetic acid anddeionized water (item 1) and homogenization is carried out for 30minutes with stirring. The crosslinking agent and the catalyst are thenadded. After homogenization for a further 30 minutes, the mixture isslowly diluted with deionized water (item 2) until the stated finalsolids content is reached. The dispersions are then subjected to vacuumdistillation for a short time, the volatile solvents being separated offfrom the distillate, as an or iic phase.

    ______________________________________                                        Dispersions     I          II      III                                        ______________________________________                                        Binder I        910                                                           Binder II       --         917     917                                        Crosslinking agent I                                                                          --         --      480                                        Crosslinking agent II                                                                         528        480     --                                         Dibutyltin dilaurate                                                                          --         --        11.2                                     Lead(II) octoate solution                                                                      33         33     --                                         (24% of Pb)                                                                   Butylglycol     --         --       70                                        Antifoam solution                                                                                1.1        1.1     1.1                                     Glacial acetic acid                                                                             30.4       29.0    29.0                                     Deionized water I                                                                             737        738     761                                        Deionized water 2                                                                             1493       2240    1154                                       Solids content (60 minutes at                                                                 30.7%      27.8%   33.4%                                      130° C.)                                                               ______________________________________                                    

PREPARATION OF THE ELECTROCOATING BATHS AND DEPOSITION OF SURFACECOATING FILMS

In order to test them as cathodic electrocoating finishes, the aqueousbinder dispersions described above are brought to a solids content of25% with deionized water. 165 parts of the pigment paste described beloware introduced into 1000 parts of the particular binder dispersion,while stirring. The solids content of the bath is then reduced to 20%(30 minutes at 150° C.) with deionized water.

PIGMENT PASTE

A paste binder is prepared according to Example 1 a) of German Laid-OpenApplication DOS 3,121,765. To do this, 200 parts of ethylene glycolmonobutyl ether are heated to 90° C. in a reaction vessel. A mixture of396 parts of N-vinylpyrrolidone, 204 parts of vinyl propionate and 1.2parts of azobisisobutyronitrile is then added dropwise in the course of2 hours. Finally, polymerization is continued for 1 hour at 90° C. Theresulting solution polymer has a Fikentscher K value of 24. A solidscontent of the copolymer solution is 76%.

250 parts of the above copolymer solution, 210 parts of ethylene glycolmonobutyl ether, 555 parts of ethylene glycol monoethyl ether, 837 partsof water, 1084 parts of kaolin, 217 parts of basic lead silicate, 145parts of carbon black, 36 parts of rutile and 3000 parts of glass beadshaving a diameter of 2 mm are stirred in a stirred ball mill for 45minutes at a speed of 1000 rpm. After the glass beads have beenseparated off, a black paste having a solids content of 50.6% isobtained.

The electrocoating baths are allowed to age for 3 days at 30° C., whilestirring. The surface coating films are deposited in the course of 2minutes at the specified voltage onto zinc-phosphatized steel specimenpanels which have been connected as the cathode. The bath temperature is27° C. The wet films deposited are washed with deionized water and bakedfor 20 minutes at 180° C. and 160° C. (Table on page 20).

    ______________________________________                                                                   Dis-                                                                 Dis-     persion Dispersion                                 Finish from       persion I                                                                              II      III                                        ______________________________________                                        Deposition voltage                                                                         V        350      300   300                                      Film thickness                                                                             μm    23       29    26                                       Ford throwing power                                                                        cm (V)   19.8     21.3  20.7                                     Leveling properties.sup.+)                                                                          2        2     1                                        Crosshatch test.sup.+)                                                                              0        0     0                                        Erichsen cupping                                                                           mm       7.4      9.8   8.2                                      Reverse impact                                                                             (in × lb)                                                                        120      160   100                                      Crosslinking.sup.++)  0        0     0                                        Baking temperature                                                                         °C.                                                                             170      170   180                                      ______________________________________                                         .sup.+) 0 best value, 5 poorest value                                         .sup.++) 20 double strokes with an MIBKimpregnated cottonwool ball            0 = surface not attacked                                                 

We claim:
 1. A method of coating a substrate by cathodicelectrodeposition of a water-dilutable binder containing latent primaryamino groups and is based on reaction products of epoxy resins andpolyamines, which contain at least one primary amino group blocked byketiminization, and, if appropriate, further primary and/or secondaryamines, wherein the binder is prepared from(A) an epoxy resin and (B) areaction product obtained by reacting a ketone with a polyamine, whichin addition to a sterically hindered primary amino group also containsat least one further primary amino group, and, if appropriate, further(C) primary and/or secondary amines.
 2. A method of coating a substrateby cathodic electrodeposition of a water-dilutable binder containinglatent primary amino groups and is based on reaction products of epoxyresins and polyamines, which contain at least one blocked primary aminogroup, and, if appropriate, further primary and/or secondary amines,wherein the binder is prepared from(A) an epoxy resin and (B) a reactionproduct which is obtained by reacting an aldehyde with a polyamine,which in addition to a sterically hindered primary amino group alsocontains at least one further primary amino group, and, if appropriate,further (C) primary and/or secondary amines.
 3. The method in claim 1 or2, wherein the amount of ketone or aldehyde used in preparation ofcomponent (B) is chosen so that all sterically unhindered primary aminogroups of the polyamine are blocked.
 4. The method of claim 1 or 2,wherein the polyamine used is an unsubstituted or substituted alicyclicor aliphatic polyamine which in addition to a sterically hinderedprimary amino group also contains a further primary amino group.
 5. Themethod of claim 1 or 2, wherein the polyamine used is an unsubstitutedor substituted alicyclic or aliphatic α-alkyl-substituted polyamine. 6.The method of claim 1 or 2, wherein the sterically hindered polyamineused is a polyamine of the formula III: ##STR4## where R4 is alkyl of 1to 5 carbon atoms, preferably methyl, ethyl or propyl, R⁵ is H or R⁴, R⁶is unsubstituted or substituted alkylene of 1 to 20 carbon atoms,preferably (CH₂)_(m) where m=1-10, and ≧
 1. 7. The method of claim 1 or2, wherein the polyamine used is 1,2-propanediamine.
 8. The method ofclaim 1 or 2, wherein the polyamine used is 1,2-diamino-2-methylpropane.9. The method in claim 3, wherein a mixture of various ketones and/oraldehydes is used in the preparation of component B.
 10. The method ofclaim 1 or 2, wherein a mixture of various polyamines is used in thepreparation of component B.